A method in which white light emitted by a light-emitting element is caused to pass through oil, and a light-receiving element is caused to detect the color of light that has passed through the oil, and thereafter the deterioration state of the oil is determined based on the output of the light-receiving element has heretofore been known as a state determining method for determining the deterioration state of machine oil (see Patent Documents 1 to 3, for example).
The state determining method disclosed by Patent Document 1 is a method in which one color that is suitable for each kind of oil is selected from among red, green, and blue components of light detected by a light-receiving element, and the deterioration state of the oil is determined based on optical absorbance with respect to the selected one color.
The state determining method disclosed by Patent Document 2 is a method in which the deterioration state of oil, i.e., the kind and amount of foreign substances that mix with oil are determined based on the difference between the optical absorbance of a red component of light detected by a light-receiving element and the optical absorbance of a blue component thereof.
The state determining method disclosed by Patent Document 3 is a method in which the state of chemical deterioration of oil is determined based on the ratio between the value of a red component of light detected by a light-receiving element and the value of a green component thereof, and the amount of foreign substances that mix with the oil is determined based on a change in optical absorbance of each of the red, green, and blue components of light detected by the light-receiving element.